Coronary hemodynamics and cardiac beating modulate atrial natriuretic factor release from isolated Langendorff-perfused rat hearts

The role of coronary hemodynamics and cardiac beating on atrial natriuretic factor (ANF) release was studied in the isolated Langendorff-perfused rat heart. ANF release was measured by radioimmunoassay. When the coronary flow rate was changed, ANF release decreased or increased in a flow-dependent manner. When the perfusion pressure was changed, ANF release also increased or decreased, respectively, with concomitant changes in coronary flow rate. Furthermore, perfusion with 50 mM potassium chloride showed immediate cardiac arrest and a decrease of ANF release to an undetectable level with a significant decrease in coronary flow. However, low but readily detectable amounts of ANF were released when coronary flow rate was maintained. These results may suggest that coronary hemodynamics and cardiac beating could be factors modulating ANF secretion from the atrium.     

In vitro study of functional parameters and cardiac arrhythmia in localized ischemia followed by reperfusion in two rat strains (Sprague Dawley and Wistar)

The cardiac parameters of two rat strains [Wistar (W) and Sprague Dawley (SD)] were compared during Langendorff perfusion. The values of coronary flow, heart rate, amplitude of contraction and the incidence of arrhythmias were studied during three 10 minutes periods: perfusion, ischemia by coronary artery occlusion and reperfusion. The values of heart rate and coronary flow of SD hearts are always higher than those of W hearts whatever the potassium (K+) concentration of perfusate (5.9 or 3 mM). Furthermore, with a high K+ perfusate (5.9 mM) and during ischemia. W rat hearts showed ventricular tachycardia periods which are never observed in SD rat hearts. It is concluded that W rat hearts present a higher sensitivity to the development of dysfunction than SD rat hearts.     

Hypoxia-induced release of atrial natriuretic factor (ANF) from the isolated rat and rabbit heart

The effect of hypoxia on the release of atrial natriuretic factor (ANF) was studied in isolated, constant-flow perfused hearts of rats and rabbits. Effluent samples were frozen pending extraction and radioimmunoassay of ANF. Hypoxia (10 min) caused a 3.9-fold (rats) and 4.6-fold (rabbits) increase of ANF release over control values. ANF release returned to control levels within 8-11 min of reoxygenation. Prolonged (20 min) hypoxia evoked further ANF release. The increase in ANF release and decrease in ventricular pressure, heart rate and coronary perfusion pressure were fully reversible, suggesting that tissues were not damaged. These results demonstrate that hypoxia induces a massive release of ANF by an as yet unexplained mechanism.     

An improved isolated working rabbit heart preparation using red cell enhanced perfusate

The performance of isolated working rabbit hearts perfused with Krebs-Henseleit (KH) buffer was compared with those in which the buffer was supplemented with washed human red blood cells (KH + RBC) at a hematocrit of 15 percent. When perfused with KH alone at 70 cm H2O afterload and paced at 240 beats/minute, coronary flow was more than double, whereas aortic flow was 40-60 percent of that in hearts perfused with KH + RBC, regardless of left atrial filling pressures (LAFP). Peak systolic pressure reached a plateau at 120 mm Hg in KH + RBC, but at 95 mm Hg in the KH group. Stroke work, however, was similar in the two groups. Despite the high coronary flow, oxygen uptake by hearts perfused with KH was substantially less and did not respond to increases in LAFP as in those perfused with KH + RBC. There was a 20 percent drop in ATP and glycogen content after 90 minutes' perfusion. In contrast, isolated hearts perfused with RBC-enriched buffer remained stable for at least 150 minutes. Irrespective of the perfusate, triacylglycerol content of the muscle remained at similar levels throughout the course of study. Increasing RBC in the perfusate from 15 percent to 25 percent had no additional effect on cardiac performance or oxygen consumption. Our findings demonstrate that in the isolated working rabbit heart inclusion of RBC in the perfusate improves mechanical and metabolic stability by providing an adequate oxygen supply.     

The effect of angiotensin II and ADH on the secretion of atrial natriuretic factor

Studies in intact animals have suggested that angiotensin II (AII) and antidiuretic hormone (ADH) increase the plasma concentration of atrial natriuretic factor (ANF). The purpose of these studies was to examine the effects of AII and ADH on ANF secretion in a rat heart-lung preparation under conditions where aortic pressure could be regulated and other indirect effects of these hormones eliminated. ANF secretion was estimated as the total amount of ANF present in a perfusion reservoir at the end of each 30-min period. A pump was used to deliver a fluorocarbon perfusate to the right atrium at rates of either 2 or 5 ml/min. In a time control series where venous return was maintained at 2 ml/min for three 30-min periods ANF secretion was 672 +/- 114, 794 +/- 91, and 793 +/- 125 pg/min (n = 6, P greater than 0.05). When venous return was increased from 2 to 5 ml/min ANF secretion increased from 669 +/- 81 to 1089 +/- 127 pg/min (P less than 0.01). The addition of AII to the perfusate in concentrations of 50, 100, or 200 pg/ml (n = 6 in each group) had no significant effect on basal ANF secretion or the ANF response to increasing venous return. Similarly, the addition of ADH to the perfusate in concentrations of 5, 25, or 100 pg/ml had no significant effect on ANF release from the heart. These results suggest that the ability of AII and ADH to increase plasma ANF concentration in vivo may be due to the effects of these hormones on right or left atrial pressure.     

Heat transfer mediated by coronary circulation in Langendorff-perfused isolated whole hearts

Although coronary flow is essential for oxygen supply, which is a prerequisite for cardiac electrical activity, energy metabolism and mechanical performance, the roles of coronary circulation on heat transfer to the heart have received less attention. This study investigated the effects of coronary circulation on epicardial temperature, the effects of temperature on coronary resistance, and the effects of ischemia on temperature fall, using isolated perfused rat or guinea pig hearts. Monophasic action potential (MAP) and epicardial temperature were recorded by a pair of suction electrodes and thermisters, while whole heart conductance (WHC) was estimated by a two-electrode instrument arranged in a diagonal array, under the alteration of the coronary flow rate of perfusate with different temperatures. MAP duration was sensitive to the local temperature, and lowering the temperature caused reduced WHC and increased coronary resistance calculated by dividing perfusion pressure by flow rate. After the onset of ischemia, WHC fell immediately in a single exponential manner, and MAP duration was abbreviated after transient behaviors explained well by the exquisite temperature gradient governed by coronary artery geometry. Epicardial temperature is maintained by coronary circulation in isolated perfused heart. Temperature-sensitive coronary tonus and MAP duration indicate that an exquisite temperature gradient underlies inhomogeneous distributions of coronary flow and electrical property. No-flow ischemia disturbs heat transfer and augments the temperature gradient transiently. Therefore, an isolated perfused heart can be considered as a heat transfer model where thermoregulation is maintained by warm coronary perfusion.     

The effect of lanthanum on the mechanical function of the isolated perfused rat heart at different extracellular calcium concentrations.

The effects of 50 microM lanthanum (La3+) on the contractile force, rate and coronary flow of rat hearts perfused with solutions containing 2.5, 5, 7.5 mM calcium (Ca2+) have been investigated. La3+ produced a rapid and marked decrease in contractile force within 1-3 min ("early La(3+)-effect"). The inhibition of contractility by La3+ was reduced progressively when the Ca2+ ion concentration in the perfusion fluid was raised from 2.5 to 7.5 mM. However, after 10-80 min of La3+ perfusion the contractile force was increased significantly ("late La(3+)-effect"). Elevation of Ca2+ during exposure to La3+ increased its effect. During the late La(3+)-effect, a marked decrease in heart rate and a significant increase in time to reach peak tension, time for half relaxation and twitch duration was observed. High concentrations of perfusate Ca2+ decreased the chronotropic response to La3+, in contrast, elevated Ca2+ potentiated La(3+)-induced increase in time to reach peak tension, time for half relaxation and twitch duration. La3+ produced a significant decrease in coronary flow. High Ca2+ augmented the decrease coronary flow. The findings indicate that La3+ may produce marked effects on myocardial function. High extracellular Ca2+ reduces the La(3+)-induced initial decrease in force of contraction, but potentiates the late increase in contractile force by La3+. Elevated external Ca2+ also increases the effects of La3+ on twitch parameters, heart rate and coronary flow.     

Effect of melatonin on the isolated heart in the standard perfusion conditions and in the conditions of calcium paradox

The effect of melatonin (MLT) on the isolated rat heart was studied using the standard perfusion conditions (Langendorff preparation) and model of calcium paradox (Ca(2+)-paradox). Ca(2+)-paradox was induced by 1 minute perfusion with Ca(2+)-free Krebs-Henseleit (KH) solution and subsequent 20 minutes perfusion with a normal Ca(2+)-containing KH solution. In MLT group, MLT (10 micromol/l) was in the perfusion solution throughout the experiment. In controls, there was no MLT. VARIABLES: heart rate, coronary flow, systolic and diastolic pressure, +dP/dt max (index of contractility) and -dP/dt max (index of relaxation) were measured at the end of stabilization, i.e. after 30 minutes of standard perfusion and then in the 5th, 10th, 15th, 20th minute after perfusion with Ca(2+)-free KH solution. RESULTS: There was no difference between MLT group and controls in the standard perfusion conditions at the end of stabilization. After perfusion with Ca(2+)-free KH solution, systolic-diastolic difference (in the 10th, 15th, 20th minute), +dP/dt max (in the 5th, l0th, 15th, 20th minute) and -dP/dt max (in the 15th minute) were significantly decreased in MLT group in comparison to controls. CONCLUSION: Melatonin didn't influence rat isolated heart in standard perfusion conditions but it made the heart more susceptible to Ca(2+)-paradox.     

Ovine atrial natriuretic factor: sequence of circulating forms and metabolism in plasma.

The sequence of ovine ANF is not known, yet sheep have been used extensively for ANF studies. We sequenced the circulating form of ovine ANF from coronary sinus plasma of sheep in paced heart failure. The main circulating form was identical to human ANF(99-126). Small amounts of ANF identical to human ANF(103-126) and ANF(101-126) peptides were also found. Incubation of labeled ANF in ovine serum suggested ANF(103-126) could be a degradation product of ANF(99-126). The endopeptidase-24.11 degradation product ANF(99-105/106-126) was not found in ovine plasma, in contrast to human plasma where it was a minor component. These results show that while the main circulating forms are similar in sheep and humans, there are differences in the minor peptides.     

Neuropeptide Y (NPY) reduces myocardial perfusion and inhibits the force of contraction of the isolated perfused rabbit heart

Isolated rabbit hearts, perfused under constant pressure (Langendorff technique) were used to study the effect of neuropeptide Y (NPY) on heart rate, force of heart contraction and rate of myocardial perfusion. No significant net change in heart rate was noted. A dose-dependent negative inotropic effect was consistently demonstrated which was characterised by slow onset and was often preceded by a transient positive inotropic response. Addition of small doses of NPY resulted in a prompt reduction in flow of the perfusate through the coronary vasculature. Since NPY is present locally in cardiac nerves, these effects may have physiological importance.     

Effects of insulin on the metabolism of the isolated working rat heart perfused with undiluted rat blood

Working rat hearts were perfused with either buffer or with defibrinated, undiluted rat blood dialyzed to remove vasoconstrictor factors. With precautions taken for sterility in the preparation of the perfusate and the apparatus, hearts were obtained which were stable as judged by stroke rate and cardiac output. In these hearts, cardiac output and coronary flow averaged 46.0 and 1.7 ml/g heart per min, respectively. Perfusion with erythrocyte-free buffer depressed cardiac output by 30%, while coronary flow averaged 8.8 ml/g of heart per min. The mean stroke rate of blood-perfused hearts was 300 beats/min but only 240 beats/min during buffer perfusion. In blood-perfused hearts, insulin did not alter stroke rate but significantly lowered coronary flow. The hormone caused a transient increase in cardiac output in hearts perfused with buffer. Insulin did not alter glucose uptake in buffer-perfused hearts but increased lactate release in perfusions with blood. Both serum fatty acids and triacylglycerol fatty acids were significant metabolic fuels in hearts perfused with undiluted blood. The preparation described would appear to be potentially useful for the study of myocardial metabolism in vitro.     

Positive inotropic, positive chronotropic and coronary vasodilatory effects of rat amylin: mechanisms of amylin-induced positive inotropy

Even though there are a few studies dealing with the cardiac effects of amylin, the mechanisms of amylin-induced positive inotropy are not known well. Therefore, we investigated the possible signaling pathways underlying the amylin-induced positive inotropy and compared the cardiac effects of rat amylin (rAmylin) and human amylin (hAmylin).Isolated rat hearts were perfused under constant flow condition and rAmylin or hAmylin was infused to the hearts. Coronary perfusion pressure, heart rate, left ventricular developed pressure and the maximum rate of increase of left ventricular pressure (+dP/dtmax) and the maximum rate of pressure decrease of left ventricle (-dP/dtmin) were measured.rAmylin at concentrations of 1, 10 or 100 nM markedly decreased coronary perfusion pressure, but increased heart rate, left ventricular developed pressure, +dP/dtmax and -dP/dtmin. The infusion of H-89 (50 μM), a protein kinase A (PKA) inhibitor did not change the rAmylin (100 nM)-induced positive inotropic effect. Both diltiazem (1 μM), an L-type Ca2+ channel blocker and ryanodine (10 nM), a sarcoplasmic reticulum (SR) Ca2+ release channel opener completely suppressed the rAmylin-induced positive inotropic effect, but staurosporine (100 nM), a potent protein kinase C (PKC) inhibitor suppressed it partially. hAmylin (1, 10 and 100 nM) had no significant effect on coronary perfusion pressure, heart rate and developed pressure, +dP/dtmax and -dP/dtmin.We concluded that rAmylin might have been produced vasodilatory, positive chronotropic and positive inotropic effects on rat hearts. Ca2+ entry via L-type Ca2+ channels, activation of PKC and Ca2+ release from SR through ryanodine-sensitive Ca2+ channels may be involved in this positive inotropic effect. hAmylin may not produce any significant effect on perfusion pressure, heart rate and contractility in isolated, perfused rat hearts.     

Intra-atrial communication and control of atrial natriuretic factor (ANF) release

Atrial natriuretic factor (ANF) release was studied in isolated perfused atria prepared from rats. When the vein-atrial junction (VAJ) was distended with an inflatable balloon, ANF release into the perfusate was greater in intact atria than in appendectomized atria. It was concluded that distention of the VAJ causes ANF release from the atrial appendage. A cascade experiment was then prepared whereby buffer from one isolated atrium perfused a second atrium. Although the VAJ of the first atrium could be distended by balloon, the atrial appendage was ligated so ANF was not secreted into the perfusate. The second atrium was intact, but no balloon was inserted. Despite the fact that there were no changes in intraluminal pressure, ANF secretion from the second atrium increased when the VAJ of the first atrium was distended. This response was blocked by the endothelin (ET) A receptor antagonist BQ-123. However, no distention-induced changes in ET-1 levels could be found in the perfusate from the first atrium. It is proposed that, in response to changes in distention of the VAJ, ANF is released remotely from the atrial appendage. The mediator does not appear to be ET-1 itself, but rather some factor that stimulates ET-1-induced ANF release within the tissue of the atrial appendage.     

Properties and regulation of a trans-plasma membrane redox system of perfused rat heart

Ferricyanide was reduced to ferrocyanide by the perfused rat heart at a linear rate of 78 nmol/min per g of heart (non-recirculating mode). Ferricyanide was not taken up by the heart and ferrocyanide oxidation was minimal (3 nmol/min per g of heart). Perfusate samples from hearts perfused without ferricyanide did not reduce ferricyanide. A single high-affinity site (apparent K_m=22 μM) appeared to be responsible for the reduction. Perfusion of the heart with physiological medium containing 0.5 mM ferricyanide did not alter contractility, biochemical parameters or energy status of the heart. Perfusate flow rate and perfusate oxygen concentration exerted opposing effects on the rate of ferricyanide reduction. A net decreased reduction rate resulted from a decreased perfusion flow rate. Thus, the rate of supply of ferricyanide dominated over the stimulatory effect of oxygen restriction; the latter effect only becoming apparent when the oxygen concentration was lowered at a high perfusate flow rate. Whereas glucose (5 mM) increased the rate of ferricyanide reduction, pyruvate (2 mM), acetate (2 mM), lactate (2 mM) and 3-hydroxybutyrate (2 mM) each had no effect. Insulin (3 nM), glucagon (0.5 μM), dibutyryl cyclic AMP (0.1 mM) and the β-adrenergic agonist ritodrine (10 μM) also had no effect, however the α₁-adrenergic agonist, methoxamine (10 μM), produced a net increase in the rate of ferricyanide reduction. It is concluded that a trans-plasma membrane electron efflux occurs in perfused rat heart that is sensitive to oxygen supply, glucose, perfusion flow rate, and the α-adrenergic agonist methoxamine.     

Dilatory and inotropic effects of corticotropin-releasing factor (CRF) on the isolated heart. Effects on atrial natriuretic peptide (ANP) release.

The effects of CRF administration on cardiac performance, coronary flow and ANP release were investigated in the rat heart. Isolated hearts were perfused at a constant filling pressure according to working heart model with a Krebs-Henseleit solution containing glucose and insulin, saturated with a gas mixture containing 95% O2 and 5% CO2. Administration of CRF via a cannula into the left atrium elicited a prolonged increase in the coronary flow rate and a transient increase in the aortic pressure resulting in an overall increase in the pressure-volume work. The oxygen consumption, after the administration of CRF, increased in accordance with the cardiac effort. No changes were observed in the spontaneous heart rate. Furthermore, administration of CRF induced a short-term increase of ANP release into the coronary perfusate. Our experiments suggest that administration of CRF produces a prolonged dilatory effect on the coronary arteries while producing a transient positive inotropic effect and a transient increase of ANP release on the isolated rat heart.     

Release of choline in the isolated heart, an indicator of ischemic phospholipid degradation and its protection by ischemic preconditioning: no evidence for a role of phospholipase D

The release of choline as a water-soluble product of phospholipid hydrolysis was measured in the perfusate of rat hearts to monitor ischemic membrane degradation and its protection by ischemic preconditioning (IPC). Hearts were subjected to global ischemia (GI; 30 min of no-flow) followed by 60 min of reperfusion. To induce IPC, GI was preceded by four no-flow episodes of 5 min each. Deleterious consequences of GI and reperfusion, namely coronary flow reduction, incidence of arrhythmias and release of cardiac troponin T, were significantly attenuated by IPC. The release of choline increased during reperfusion in a biphasic manner: a first phase peaked immediately after GI and was followed by a second, delayed phase indicating choline release caused during reperfusion. Only the second phase was blocked by both IPC and by AACOCF3 (5 microM), an inhibitor of cytosolic phospholipase A2. The activity of phospholipase D (PLD) was unchanged after GI or IPC or GI plus IPC. In conclusion, choline release into heart perfusate was found to be a useful real-time indicator of phospholipid degradation caused by GI and by reperfusion and its protection by IPC. The results supplement previous observations on the accumulation of fatty acids in the phospholipid pool. There was no evidence for PLD activation by GI or IPC.     

Carboxypeptidase B and other kininases of the rat coronary and mesenteric arterial bed perfusates

We describe the enzymes that constitute the major bradykinin (BK)-processing pathways in the perfusates of mesenteric arterial bed (MAB) and coronary vessels isolated from Wistar normotensive rats (WNR) and spontaneously hypertensive rats. The contribution of particular proteases to BK degradation was revealed by the combined analysis of fragments generated during incubation of BK with representative perfusate samples and the effect of selective inhibitors on the respective reactions. Marked differences were seen among the perfusates studied; MAB secretes, per minute of perfusion, kininase activity capable of hydrolyzing approximately 300 pmol of BK/min, which is approximately 250-fold larger amount on a per unit time basis than that of its coronary counterpart. BK degradation in the coronary perfusate seems to be mediated by ANG I-converting enzyme, neutral endopeptidase 24.11-like enzyme, and a dl-2-mercaptomethyl-3-guanidinoethylthiopropanoic acid-sensitive basic carboxypeptidase; coronary perfusate of WNR contains an additional BK-degrading enzyme whose specificity resembles that of neurolysin or thimet oligopeptidase. Diversely, a des-Arg(9)-BK-forming enzyme, responsible for nearly all of the kininase activity of MAB perfusates of WNR and spontaneously hypertensive rats, could be purified by a procedure that involved affinity chromatography over potato carboxypeptidase inhibitor-Sepharose column and shown to be structurally identical to rat pancreatic carboxypeptidase B (CPB). Comparable levels of CPB mRNA expression were observed in pancreas, liver, mesentery, and kidney, but very low levels were detected in lung, heart, aorta, and carotid artery. In conclusion, distinct BK-processing pathways operate in the perfusates of rat MAB and coronary bed, with a substantial participation of a des-Arg(9)-BK-forming enzyme identical to pancreatic CPB.     

Mechanosensitive release of parathyroid hormone-related peptide from coronary endothelial cells

10.1152/ajpheart.00925. 2001.-Parathyroid hormone-related peptide (PTHrP) is expressed throughout the cardiovascular system and is able to dilate vessels. This study investigated whether mechanical forces generated by changes in regional perfusion influence PTHrP release from the coronary vascular bed. Experiments were performed in vitro on saline-perfused rat hearts or isolated coronary endothelial cells exposed to cyclic strain and in vivo in anesthetized pigs. In vitro, PTHrP release from saline-perfused rat hearts was strongly correlated with coronary flow (r = 0.84). Increasing coronary flow from 5 to 10 ml/min increased PTHrP release from 442 +/- 42 to 1,563 +/- 167 pg/min. Increasing the viscosity of the perfusate did not change basal PTHrP release. Increasing flow without a concomitant increase in pressure did not lead to an increase in release rate, but reduction in pressure under flow-constant conditions reduced PTHrP release rate. Cyclic strain induced a strain-dependent release of PTHrP from endothelial cells that was inhibited by the addition of a calcium-chelating agent. In vivo, there was a net release of PTHrP in the coronary circulation and decreases in coronary flow and pressure decreased the PTHrP release rate. Bradykinin in the presence of constant pressure increased PTHrP release, probably by increasing the intracellular calcium concentration in coronary endothelial cells. In summary, mechanical forces evoked by blood flow can trigger a constant PTHrP release.     

Hemodynamic action of calcitonin gene-related peptide in the isolated rat heart

The effects of calcitonin gene-related peptide (CGRP) on heart rate, coronary flow, pressure development, and time to ischemic contracture were studied in the isolated, perfused rat heart. A bolus of CGRP (2640 pmols) caused significant increases in heart rate and coronary flow; these effects were sustained for at least five minutes after injection. The increase in coronary flow was independent of heart rate, since CGRP caused an increase in coronary flow in non-beating (potassium-arrested) hearts. The dose-response of CGRP was studied using five doses (65, 218, 658, 1320 and 2640 pmols) given as bolus injections. Although the increase in heart rate was apparently dose-dependent, significant increases above baseline were observed only with the two highest doses. In contrast, coronary flow increased significantly above baseline with the injection of all but the lowest dose of CGRP. Ten minutes after injection of CGRP, all hearts were made ischemic. The time to onset of ischemic contracture was approximately 11 minutes for those hearts that received 65 pmols of CGRP; however, for those hearts receiving all other doses of CGRP, the time to onset of contracture was approximately 8 minutes. We conclude that CGRP significantly decreases the resistance of the coronary vascular bed, and that it may be an important regulator of regional blood flow in the heart.     

Flow-induced oxygen uptake by the perfused rat hindlimb is inhibited by vasodilators and augmented by norepinephrine: a possible role for the microvasculature in hindlimb thermogenesis

Oxygen uptake in the perfused rat hindlimb was studied at 25 degrees C using an artificial perfusate, and the effects of perfusate flow rate, norepinephrine, and vasodilators were compared. Hindlimb oxygen uptake and perfusion pressure each increased as the flow rate was increased stepwise from 2 to 18.5 mL/min per hindlimb. At each flow rate, the rate of oxygen uptake was inhibited by the vasodilator nitroprusside (0.5 mM) and increased by norepinephrine (5 nM). A corresponding change in perfusion pressure also occurred, with norepinephrine leading to a marked increase and nitroprusside leading to a decrease; however, changes in oxygen uptake and pressure were not linearly related. The lactate/pyruvate ratio of the perfusate was used as an index of tissue perfusion and was determined at each flow rate. Lactate and pyruvate efflux increased as the flow rate was increased stepwise from 2 to 18.5 mL/min per hindlimb. At 2 mL/min per hindlimb, the lactate/pyruvate ratio was 15; at flow rates equal or greater than 4 mL/min per hindlimb, the ratio was constant at 9. Nitroprusside had no significant effect on the ratio at any flow rate even though a marked inhibitory effect on oxygen uptake was evident. Muscle content of high energy phosphates at 8 mL/min per hindlimb did not differ before and after treatment with vasodilators. In addition, the vasodilators had no apparent effect on skeletal muscle oxygen uptake or force development during electrical stimulation. The findings indicate that oxygen uptake by the hindlimb is not limited by inadequate perfusion and that oxygen uptake can be further increased by norepinephrine.(ABSTRACT TRUNCATED AT 250 WORDS)